Bearing in mind this is a book for learners ... in the sense that all the poems are either to help learners, or are about the learning experience, it's a superb irony that the title is pretty enigmatic.
The thing about it is, well, the bits... the "er", the " fod" can mean different things, and so it's hard to pin down what the whole phrase means ...especially when a lot of expressions like that don't produce a literal meaning .. like these ...
i fod (i ) = supposed to
am fod = because
er cof = in memory of ...
er mwyn = for the sake of ....
Google translate says " er dy fod" means " although you."
But ... google translate is hardly reliable.... but you can see where it's coming from.
There is a poem in the book called " Er dy fod" ( p48) ... but it doesn't help.... much.
So .. I decided to make a " corpus" ..that's a whole collection of examples where a word or phrase is used ... to get some idea about its meaning.
I didn't find very much though, especially as when you type " er dy fod" into search engines you mainly get articles about "nerdy food" whatever that is. But I did find a few ... and I got google translate to have a stab at translating them, despite its frequent imperfections ...
[a] Er dy fod yn ddig, dangos drugaredd aton ni!
Although you are angry, show mercy to us
[b] Diolch i ti, er dy fod di’n frenin, dy fod di hefyd yn ffrind i ni.
Thank you, for that you are a king, that thou also our friend,
[c] Ac er dy fod di'n blino ...
And even though you're tired ...
[d[ Er dy fod angen pobl eraill i’th helpu i ...
Although you need your helping others to ...
This one's from Bryn Fon , the "Coedwig Ar Dan" lyrics
[e] ond er dy fod yn pell i fwrdd dwi'n teimlo gwres dy gariad
but even though you are far away I feel your love heat.
Crumbs! Get a room !
What these show is that "er dy fod" is a bit fluid in meaning, but generally might mean ...
although
even though
for that you are .... which approximates to " even though"
So ... the power of the corpus has been demonstrated. All hail to the corpus.
You don't have this trouble with French you know.
And now, here's that Bryn Fon song ... just for the record of course ...